1. Field of the Invention
This invention relates to switched-capacitor circuitry such as is found in sigma-delta analog-to-digital and digital-to-analog converters. More particularly, this invention relates to an IC chip having switched-capacitor circuitry provided with means to allow the circuit gain to be increased without increasing the magnitude of the d-c offset signal in the output.
2. Description of the Prior Art
Converters of the sigma-delta type have become quite popular for use in high performance audio systems. Among their many and varied applications, such converters have found considerable use in producing high-fidelity digital-audio signals to be supplied to personal computers (PCs) where the digital data may be processed (e.g., compressed), stored, and played back. It is important in such audio systems that the digital-audio signals be very precise representations of the original analog-audio signals, substantially free of distortion. This capability is a feature of ADCs employing switched-capacitor circuitry and sigma-delta modulation. Such audio systems also commonly make use of DACs as well as ADCs employing switched-capacitor circuitry and sigma-delta modulation.
The amplifiers in the signal chain of such a high quality sound system should be arranged so that the amplifier d-c offset signals, and any other unwanted d-c signals, are not gained along with the digital sound signals. Otherwise, such d-c signals can use up an excessive portion of the dynamic range of the circuitry, and prevent satisfactory operation at high gain settings due to saturation.
Avoiding such gaining of the offset and other d-c signals can be achieved by the use of a-c coupling capacitors. However, it has been found that such a-c coupling must be incorporated in switched-capacitor circuitry in a special way to avoid problems in practical usage.
One problem stems from the fact that the coupling capacitor must be relatively large, and for that reason should be located off-chip. Thus, if the circuit arrangement provides simply that the coupling capacitor be in series with the internal signal path on the chip, as might be thought proper where multiple audio inputs must be served, it would be necessary to provide additional pins on the IC chip to make electrical connections between the capacitor and the circuit nodes to which the capacitor is to be connected. This adds significant expense to the manufacture of the IC part.
Still other practical problems are encountered in designing the converter circuitry to achieve the desired results. For example, the input circuitry of the converter desirably should include one and preferably more buffer amplifiers preceding the switched-capacitor circuitry forming part of the converter, and the d-c coupling of such buffer amplifiers can cause harmful interaction with gain-adjustment circuitry following the buffer amplifier. The design should accommodate the use of buffer amplifiers without adverse d-c offset signal effects from interaction with the following circuitry.